All surface-excitation studies of Au in the past focused on the well-known 2.4 eV surface plasmon polariton in the visible spectral regime. The existence of surface exciton polaritons is believed to be pristine to the spectral regimes, showing strong excitonic absorptions [F. Yang et al., Phys. Rev. Lett. 64, 559 (1990); Phys. Rev. B 44, 5855 (1991)]. The presence of surface exciton polaritons in far-UV in Au (10>eV), where the optical and electronic properties of Au are dominated by broad interband transitions that display characters of rather weak and diffused excitonic oscillator strengths, is not expected and has never been discussed. Reexamining the reports of Yang et al. and using electron energy-loss spectroscopy with a 2 Å electron probe in aloof (optical near-field) setup and real-space energy-filtered imaging, we firmly establish the existence of surface exciton polaritons in individual Au nanoparticles in the far-UV spectral regime. These results indicate that surface exciton polaritons indeed can be excited in weak excitonic onsets in addition to their general believing for the sharp excitonic oscillations. Our experimental observations are further confirmed by the theoretical calculations of electron energy-loss spectra. The unmatched spatial resolution (2 Å) of the electron spectroscopy technique enables an investigation of individual nanomaterials and their surface excitations in aloof setup. The surface exciton polaritons in individual Au nanoparticles thus represent an example of surface excitations of this type beyond the visible spectral regime and could stimulate further interests in surface exciton polaritons in various materials and applications in novel plasmonics and nanophotonics at high energies via manipulations of the associated surface near fields.
Physical Review B (Condensed Matter and Materials Physics) 77(24), 245402(9pages)